Donald Trump tried to put a difficult week behind him yesterday as he neared today's Republican presidential contest in Wisconsin, where he is campaigning from the unfamiliar position of clearly being the underdog.

Donald Trump tried to put a difficult week behind him yesterday as he neared today's Republican presidential contest in Wisconsin, where he is campaigning from the unfamiliar position of clearly being the underdog.

The Republican front-runner is at risk of losing the Midwestern state to US Senator Ted Cruz of Texas, an outcome that would dent the New York billionaire's aura of invincibility and make it harder for him to win the vital 1,237 delegates needed for the party's nomination for the November 8 election.

On the Democratic side, Senator Bernie Sanders of Vermont is trying to protect his lead over front-runner Hillary Clinton in opinion polls in Wisconsin and to eke out another victory over the former secretary of state.

Trailing Mr Cruz in the polls in Wisconsin, Mr Trump spent the entire weekend campaigning in the state and planned to draw in his wife, Melania, yesterday. He stayed on message, telling supporters in West Allis, Wisconsin, that Mr Cruz was a liar and a "dirty rotten cheater" who is weak on immigration and would cut Social Security benefits.

"Wisconsin is going to be such a big surprise. We are doing so well," Mr Trump said.

The New York real estate tycoon has won 20 presidential nominating contests and leads in the delegate count that will determine the Republican Party's nominee. But he trails Mr Cruz by 10 percentage points in some Wisconsin polls.

A loss would add to Mr Trump's woes after his campaign was rocked last week by the fallout from his suggestion, which he later dialled back, that women be punished for getting abortions if the procedure is banned. Uncharacteristically, Mr Trump also acknowledged that he made a mistake retweeting an attack on Mr Cruz's wife, according to 'The New York Times'. He also drew fire last week for saying he would not rule out using nuclear weapons in Europe and that Japan and South Korea might need their own nuclear arsenals to ease the US financial commitment to their security.

More Republicans are recognising that "nominating Donald Trump would be a train wreck," Mr Cruz said in Green Bay, Wisconsin. Mr Cruz faces difficulty in winning the delegates needed to secure the nomination, given that the next states to vote, including New York on April 19, are Trump-friendly territory.

Ms Clinton is already eyeing New York, holding campaign stops there yesterday even as other candidates make their final pitches in Wisconsin.

"I'm absolutely confident I will be the nominee," Ms Clinton told ABC in an interview that aired yesterday as she and Mr Sanders continued to spar over scheduling more debates.

Sanders adviser Tad Devine said the senator wanted another prime-time debate with Clinton.

"If we can continue to win, if he has a good day tomorrow, we're going to make his case through New York all the way to California," Mr Devine said.

Irritant

Republicans Mr Cruz and Ohio Governor John Kasich, who is third in the race, want to deny Mr Trump enough delegates so that the nominee is determined at the Republican National Convention in July.

Over the weekend, Mr Trump complained that Mr Kasich was an irritant gobbling up some of the delegates Mr Trump needs.

"The problem is he's in the way of me, not Cruz," Mr Trump said.

Mr Kasich, who has vowed to stay in the race, tweeted: "That's not how our republic works, Donald. We'll keep fighting until someone reaches a majority of delegates."

Yesterday, Republican National Committee strategist Sean Spicer said on CNN that the party would "never tell any candidate to get in or out of the race".